In recent years, a plate display device has been widely applied in a display device field. This is because a recent plate display device can be manufactured small and/or thin easily. Since a liquid crystal display (LCD) device can easily realize multicolor display, the LCD device is often used as such a plate display. A transmissive LCD device generally includes a backlight device so as to ensure luminance of a display surface. Further, a transflective LCD device, which has functions of both a reflective LCD device and a transmissive LCD device, also includes a backlight device as a light source for a transmissive mode.
Such a backlight device has a function of adjusting luminance (i.e. light control function), so as to improve quality of a display screen, or suppress electric power consumption in driving a display device with a battery. A cold cathode florescent tube (hereinafter, referred to as “fluorescent lamp”) is generally used as a light source of the backlight. There have been known, for example, the following light control methods for the backlight using a cold cathode fluorescent lamp: (i) a pulse width modulation (PWM) light control method in which a pulse width of a pulse voltage which is applied at a certain cycle is controlled; and (ii) a tube current light control method in which a current flowing in a florescent lamp is controlled.
In recent years, a backlight device adopting both the aforementioned light control methods has been proposed. For example, Patent Literature 1 discloses a technique of controlling light in such a manner that (i) in a low luminance region, light is controlled by only the pulse width modulation, while a tube current is maintained at a certain value, and (ii) in a high luminance region, in which the pulse width exhibits a duty ratio of not less than 100%, light is controlled by adjusting the tube current. FIG. 15 is a graph showing a change in luminance in a backlight device disclosed in Patent Literature 1. With the arrangement, in the high luminance region, it is possible to change the tube current in a range from a minimum value to a maximum value while the pulse width is maintained at a duty ratio of 100%. Therefore, it is possible to control light in a wider range.